Gaseous discharge lamp with stabilizing arrangement



S. SOLOW March 28, 1967 Filed Aug. 27, 1965 I l x R mw mm m m, a A m a m m M, sy

United States Patent Otlice 3,3i 1,?75 Patented 28, 1967 3,311,775 GASEGUS DISCHARGE LAMP WETH STABILEZHQG ARRANGEMENT Samuel Solow, Roiiing Hills, Calii, assignor, by mesue assignments, to GTC (Zorporation, a corporation of Texas Fiied Aug. 27, 1963, Ser. No. 364,378 9 Claims. (Cl. 313-161) This invention relates to electrodeless gaseous discharge lamps and, more particularly, to improved arrangements of such lamps providing an increased stability in operation.

An electrodeless gaseous discharge tube or lamp, sometimes referred to as a glow lamp, is one which contains a gas or vapor whose atoms are excited by an oscillating electric field to produce light. Optical pumping lamps employing electrodeless discharge tubes are used in gas cell type atomic frequency standards, magnetometers, atomic gyroscopes, and the like. In use the electrodeless discharge tube is driven by an applied electromagnetic field at a selected frequency in order to produce the desired optical discharge.

The electrodeless discharge lamps are so-called because they do not employ any internally mounted electrodes to initiate or maintain the optical discharge. Rather, the driving electromagnetic field is induced within the discharge region of the tube from an externally positioned generator which is commonly in the form of a solenoid encompassing a portion of the discharge tube and which may be energized at a radio frequency. Discharge tubes of this type usually contain some quantity of inert gas such as argon or neon and a small amount of alkali metal raised to a temperature above the melting point of this metal. The contained gases are electrically excited to provide an ionized plasma which is the source of the optical pumping radiation.

In many applications of these devices, it is virtually essential that the optical radiation produced be steady and not fluctuate as a function of time. In the usual utilization device, an optical pumping lamp incorporates an electrodeless discharge tube within a parabolic reflector for proper concentration and direction of the generated light. Most of the useful light derived from such an arrangement originates in the near vicinity of the focal point of the parabolic reflector. It is most desirable that the light originate at a point source located at the focus of the parabola in order that a beam of collimated light be provided. Should the plasma vary in position within the electrodeless discharge lamp during operation, the light which is available from the parabolic reflector will fiuctuate, depending upon the position relative to the focus, with the adverse result of an unwanted variation in light intensity at any particular point within the beam.

Although electrodeless discharge lamps are in common use in the applications mentioned above, many workers in the field have found these lamps prone to plasma oscillations which, in turn, cause the light to fluctuate in intensity at any given point of observation along the plasma. Various attempts have been made to deal with this particular problem, none of which have been particularly successful. One approach has been to drive the electrodeless discharge lamp at increased power levels in order to minimize the adverse effect of plasma oscillations. This, however, unfortunately diminishes the useful life of the discharge tube, due to excessive heat and ion bombardment, and also necessitates the use of higher powered oscillators for driving the lamp. Another approach is to energize the lamp with electromagnetic radiation at higher frequencies. This introduces the difficulty of generating the desired radiation at very high frequencies and does not entirely eliminate the plasma oscillations.

Accordingly, it is a general object of the present invention to provide an improved arrangement of an electrodeless discharge lamp. 7

It is a more specific object of the present invention to eliminate the undesirable plasma oscillations which are encountered in operating an electrodeless discharge lamp.

A further object of the present invention is elimination of plasma oscillations encountered in an electrodeless discharge lamp without the necessity of resorting to higher power or higher frequency radiation for energizing the lamp.

An additional object of the present invention is to provide an electrodeless discharge lamp capable of operation with improved stability over an extended period at reduced power input.

In brief, the present invention comprises an electrodeless discharge lamp incorporating a discharge tube which is in the general form of a right circular cylinder with closed ends. In accordance with an aspect of the invention, the tube includes a prod, preferably of the same material as the walls of the tube for ease of fabrication, extending coaxially from one end of the tube toward the center thereof. This prod is configured so that it extends into the plasma which is ionized when the tube is in operation but terminates short of the focal point of the associated parabolic reflector in which the tube is mounted.

In one particular arrangement in accordance with the invention, the tube and the enclosed prod are fabricated of glass, and rubidium vapor, along with an inert gas such as argon or neon, are utilized as the active elements within the tube. The provision of the glass prod as described, in accordance with an aspect of the invention, effectively eliminates the undesired plasma oscillations encountered in previously known devices of this type. The theoretical basis of this improved result is not fully understood; however, it is hypothesized that the prod may act as a damper to electro-acoustical oscillations or it may upset the thermal gradients within the tube so that the conditions for the undesired plasma oscillations are nullified.

In addition, in one particular arrangement in accordance with the invention, an electrode is aflixed to the outside of the tube adjacent the base of the prod and a pulse source is connected thereto which is capacity coupled through the glass to trigger the ionization of the gases within the tube to initiate operation of the electrodeless discharge lamp. As is common with lamps of the type involved in the present invention, the lamp is used in conjunction with a metallic reflector which is commonly connected to ground or a point of reference potential. In the present arrangement of the invention, this connection effectively completes the capacitive circuit for the triggering pulse from the pulse source. This particular arrangement advantageously permits operation of the lamp at significantly reduced power levels, thus extending the useful life of the device and permitting a reduction in the size and cost of the auxiliary equipment needed to drive the lamp. The higher dielectric constant of the glass prod relative to the surrounding plasma within the tube more effectively carries the high voltage trigger pulse which is applied to the rear of the lamp to the interior of the tube and therefore makes the lamp easier to ignite. As is known, the dielectric material of the glass prod effectively pro vides a coupling path for the triggering pulse to the interior of the lamp tube. Although the glass prod may be of the same material and dielectric constant vof the walls of the tube envelope, this merely means that multiple paths exist which provide somewhat diiferent impedances for the triggering pulse as it is capacitively coupled between the electrode 'afiixed to the outside of the tube adjacent the base of the prod and the lamp reflector.

A better understanding of the invention may be had from a consideration of the following detailed description, taken in conjunction with the accompanying drawing, in which:

FIG. 1 is a sectional representation of an electrodeless discharge lamp in accordance with the present invention; and

FIG. 2 is a cross-sectional view taken along the line 2-2 in the lamp shown in FIG. 1.

As represented in FIG. 1, an electrodeless discharge lamp is shown comprising a discharge tube or bulb 12 mounted within a ferrule 14 which in turn is aflixed to a parabolic reflector 16. A solenoid winding 18 is represented surrounding the right-hand end of the tube 12 for applying an electromagnetic field to ionize the gases within the tube 12. Such a solenoid is conventionally driven from an associated RF generator by leads which are connected through the holes of the parabolic reflector 16, but these connections and associated driving equipment have been omitted for the sake of simplicity. A typical circuit arrangement for operating an electrodeless discharge lamp 10 as thus for described is shown in FIG. 1 of the W. A. Marrison Patent 2,974,243. The point 17 represents the focal point of the parabolic reflector from which various rays of optical radiation are represented as emanating to form the desired beam of light after reflection from the interior surface of the parabolic reflector 1s. A prod 13 is shown extending interiorily from one end of the tube 12 to terminate short of the focal point 17. At the left-hand end of the tube 12 an electrode is shown adjacent the base of the prod 13. The electrode 20 may be affixed to the tube 12 by a quantity of silver epoxy resin 21. A pulse source 24 is shown connected to the electrode 20.

FIG. 2 represents a cross-sectional view of the tube 12 taken along the lines 2-2 of FIG. 1 to show the generally circular configuration of the cross section of the tube 12 and its prod 13.

In operation, oscillating current of a suitable frequency is applied to the solenoid 18 and a trigger pulse is transmitted from the pulse source 24 to the electrode 20 at the base of the prod 13. This pulse is transmitted to the interior of the tube 12 by the prod 13 and effectively serves to initiate ionization of the gases therein which are thereafter maintained as an ionized plasma by the oscillating electromagnetic field from the solenoid 18. During the continued operation of the lamp 10, the prod 13 advantageously serves to maintain the ionized plasma, represented by the outline 22, in a stable configuration so that the light which is present at the focus 17 is maintained virtually constant in intensity. Since the base of the prod 13 is the coolest spot in the tube 12, the excess vapor within the lamp tends to condense adjacent this point so that the random formation of droplets of the contained vapor along the walls of the tube 12 no longer occurs. Thus this arrangement of the present invention additionally serves to eliminate a further source of interference with the purity and stability of the ionized plasma light source.

It will be appreciated that, although the depicted arrangement of the invention includes a pulse source 24 and appropriate connections for applying a triggering pulse to initiate plasma ionization, such auxiliary triggering equipment is not essential. The advantageous effects derivable from the stabilizing prod of the described arrangement of the invention may be realized in arrangements utilizing other means for triggering ionization in the tube.

It has been found that the advantageous results produced by providing a prod extending from one end of an electrodeless discharge lamp in accordance with the present invention are dependent to some extent on the dimensions of the prod and its spacing from the center of focus of the associated parabolic reflector. In one preferred arrangement of the invention, an electrodeless discharge tube is fabricated of Pyrex glass and filled with rubidium vapor plus an inert gas before sealing. The tube is formed with a glass prod extending from one end thereof. The various dimensions of this preferred embodiment of the invention are as follows:

Inches Overall length 1.50 Outside diameter of tube .281 Wall thickness .039 Length of prod .62

The tube 12 is cemented within the ferrule 14 by silver epoxy which is cured at C. for thirty minutes. The electrode 20 is formed by making a loop from one strand of a coaxial cable. The loop is cemented to the end of the tube 12 by silver epoxy which is then cured at C. for one hour. The arrangement is then fitted within the parabolic reflector 16. The outer end of the prod 13 is approximately 0.1 inch from the focus of the reflector 15.

Although there has been shown and described herein one particular arrangement of an electrodeless discharge lamp in accordance with the present invention in order to illustrate the preferred embodiment thereof, it will be appreciated that the invention is not limited thereto. Accordingly, any and all modifications, variations or equivalent arrangements falling within the scope of the annexed claims should be considered to be a part of the invention.

What is claimed is:

1. An electrodeless discharge lamp comprising a discharge tube closed at both ends and consisting of a dielectric material, a solenoidal coil positioned around a portion of the discharge tube for establishing an electromagnetic field therein, and stabilizing means including a prod consisting essentially of the same material as the discharge tube afiixed to one of the closed ends of said tube and extending within the discharge tube for maintaining the intensity of light generated in the tube substantially constant at a predetermined point therein.

2. An electrodeless discharge lamp comprising a discharge tube closed at bath ends, a solenoidal coil positioned around a portion of the discharge tube for establishing an electromagnetic field therein, stabilizing means including a non-conductive prod of a dielectric material aifixed to one of the closed ends of said tube and extending within the discharge tube for maintaining the intensity of light generated in the tube substantially constant at a predetermined point therein, and means coupled to the prod for initiating an electrical discharge within said tube.

3. An electrodeless discharge lamp in accordance with claim 2 wherein said last mentioned means comprises a terminal afiixed to said tube adjacent the prod and a trigger pulse source connected to said terminal.

4. An electrodeless discharge tube for use as a light source comprising a glass cylinder closed at both ends and containing a quantity of rubidium vapor, means for initiating and maintaining an ionized plasma within said tube, and means for stabilizing the ionized plasma to minimize oscillations thereof comprising a substantially solid, glass prod extending interiorly of said tube from one of the closed ends thereof along the axis of the cylinder.

5. An electrodeless discharge tube for use as a light source comprising a glass cylinder closed at both ends to contain a predetermined quantity of rubidium vapor and an inert gas, a substantially solid, glass prod extending interiorly from one of the closed ends of said tube concentrically positioned therein, and an electrical connection atfixed to said tube adjacent the base of said prod for facilitating the initiation of an ionic discharge within the tube.

6. An electrodeless discharge tube for use as a controlled source of light from a focal point thereof comprising a substantially transparent symmetrical envelope closed at both ends to contain a predetermined quantity of an ionizable material, said material being subject to plasma oscillations when ionized by an applied oscillating electromagnetic field, and means for minimizing said plasma oscillations comprising a substantially solid prod extending inwardly from one of the closed ends along the major axis of the tube concentrically therewith to a point short of the focal point of the tube.

7. An electrodeless discharge tube for use as a light source comprising a substantially transparent cylindrical envelope of a dielectric material closed at both ends to enclose an ionizable material, means for applying an oscillating electromagnetic field to maintain an ionized plasma once established within said tube, and a dielectric prod consisting essentially of the same material as the cyclindrical envelope extending into said tube concentrically therewith from one of the closed ends for both transmitting an applied trigger pulse to initiate ionization of the enclosed vapor and for stabilizing the resulting ionized plasma to maintain a constant light intensity at a predetermined point within the tube.

8. An electrodeless discharge lamp for use as a light DAVID J. GALVIN,

source comprising a reflector having a fixed focal point, a sealed glass tube closed at both ends and mounted therein and containing an ionizable material, means for initiating and maintaining an ionized plasma Within said tube, and means for stabilizing the ionized plasma to minimize oscillations thereof comprising a solid, glass prod extending interiorly of said tube from one of the closed ends thereof along the axis of the tube and terminating a predetermined distance from the focal point of the reflector.

9. An electrodeless discharge lamp in accordance with claim 8 wherein the overall length of the tube is approximately 1.50 inches, the length of the prod is approximately .62 inch and the focal point of the reflector is approximately 0.1 inch from the end of the prod.

References Cited by the Examiner UNITED STATES PATENTS 1,698,691 1/1929 Buttolph 315-248 X 2,974,243 3/1961 Morrison 313113 X 3,122,676 2/1964 Kind 315248 X 3,138,739 6/ 1964 Farmer 315248 3,227,923 1/ 1966 Marrison 315248 Primary Examiner. GEORGE N. WESTBY, D. E. SRAGOW, Examiners. 

1. AN ELECTRODELESS DISCHARGE LAMP COMPRISING A DISCHARGE TUBE CLOSED AT BOTH ENDS AND CONSISTING OF A DIELECTRIC MATERIAL, A SOLENOIDAL COIL POSITIONED AROUND A PORTION OF THE DISCHARGE TUBE FOR ESTABLISHING AN ELECTROMAGNETIC FIELD THEREIN, AND STABILIZING MEANS INCLUDING A PROD CONSISTING ESSENTIALLY OF THE SAME MATERIAL AS THE DISCHARGE TUBE AFFIXED TO ONE OF THE CLOSED ENDS OF SAID TUBE AND EXTENDING WITHIN THE DISCHARGE TUBE FOR MAINTAINING THE INTENSITY OF LIGHT GENERATED IN THE TUBE SUBSTANTIALLY CONSTANT AT A PREDETERMINED POINT THEREIN. 